Transient natural convection: scale analysis of dry cooling towers

Journal of Thermal Analysis and Calorimetry - Transient free convection from a heated surface immersed in a cold ambient is investigated using scale analysis. The heated plate is our model for the...     

Viscous oil–water flows in a microchannel initially saturated with oil: Flow patterns and pressure drop characteristics

Immiscible viscous liquid–liquid two-phase flow patterns and pressure drop characteristics in a circular microchannel have been investigated. Water and silicone oil with a dynamic viscosity of 863 mPa s were injected into a fused silica microchannel with an inner diameter of 250 μm. As the microchannel was initially filled with the silicone oil, an oil film was found to always form and remain on the microchannel wall. Different flow patterns were observed and classified over a wide range of water and oil flow rates. A flow pattern map is presented in terms of Re, Ca, and We numbers. Two-phase pressure drop data have also been collected and analyzed to develop a simple correlation for slug, annular and annular-droplet flow patterns in terms of superficial water and oil velocities.     

A perturbation solution for forced convection in a porous-saturated duct

Fully developed forced convection through a porous medium bounded by two isoflux parallel plates is investigated analytically on the basis of a Brinkman–Forchheimer model. The matched asymptotic expansion method is applied for small values of the Darcy number. For the case of large Darcy number the solution for the Brinkman–Forchheimer momentum equation is found in terms of an asymptotic expansion. With the velocity distribution determined, the energy equation is solved using the same asymptotic technique. The results for limiting cases are found to be in good agreement with those available in the literature and the numerical results obtained here.     

Vacuum sampling in the landscape during inflation

We consider the phenomenological consequences of sampling multiple vacua during inflation motivated by an enormous landscape. A generic consequence of this sampling is the formation of domain walls, characterized by the scale mu of the barriers that partition the accessed vacua. We find that the success of big bang nucleosynthesis (BBN) implies mu > or = 10 TeV, as long as the sampled vacua have a nondegeneracy larger than O(MeV4). Otherwise, the walls will dominate and eventually form black holes that must reheat the universe sufficiently for BBN to take place; in this case, we obtain mu > or = 10(-5)MP. These black holes are not allowed to survive and contribute to cosmic dark matter density.     

LAMINAR DISSIPATIVE FLOW IN A POROUS CHANNEL BOUNDED BY ISOTHERMAL PARALLEL PLATES

IntroductionTheproblemofforcedconvectioninaporousmediumchannelorductisaclassicalone (atleastforthecaseofslugflow (Darcymodel) .Therehasrecentlybeenrenewedinterestintheproblembecauseoftheuseofhyperporousmediainthecoolingofelectronicequipment.Recently ,NieldandBejan[1]refertomorethan 3 0papersonthetopic ,butnoneofthemdealsexplicitlywiththecaseofthermaldevelopment.ThisgapintheliteraturehasbeenpartlyfilledbyNieldetal.[2 - 4 ].Lahjomrietal.[5 ,6 ]havesolvedmathematicallysimilarproblemsbyusingthe…     

Genetic algorithm optimization for finned channel performance

Compared to a smooth channel,a finned channel provides a higher heat transfer coefficient;increasing the fin height enhances the heat transfer.However,this heat transfer enhancement is associated with an increase in the pressure drop.This leads to an increased pumping power requirement so that one may seek an optimum design for such systems.The main goal of this paper is to define the exact location and size of fins in such a way that a minimal pressure drop coincides with an optimal heat transfer based on the genetic algorithm.Each fin arrangement is considered a solution to the problem (an individual for genetic algorithm).An initial population is generated randomly at the first step.Then the algorithm has been searched among these solutions and made new solutions iteratively by its functions to find an optimum design as reported in this article.     

A molecular dynamics study on the role of attractive and repulsive forces in excess heat capacity at constant volume of dense fluids

The curves of experimental heat capacity against density show a minimum around and below the critical temperature (Tc), but at higher temperatures, this minimum is not observed. In this study, the role of attractive and repulsive forces on excess heat capacity of Lennard–Jones (LJ) dense fluids has been investigated using a molecular dynamics simulation technique. LJ potential is divided into attractive and repulsive parts. From the molecular dynamics calculations, potential energy and heat capacities have been obtained for Argon at temperatures of 100–500?K. The repulsive forces play the main role in causing the heat capacities at temperatures greater than critical point. Around and below the critical temperature, the role of repulsion is dominant at high densities, but attraction has the main role at low densities, consequently at middle densities, a minimum is formed.     

Experimental and thermodynamic modelling of systems containing water and ethylene glycol: Application to flow assurance and gas processing

Accurate knowledge of hydrate phase equilibrium in the presence of inhibitors is crucial to avoid gas hydrate formation problems and to design/optimize production, transportation and processing facilities. In this communication, we report new experimental dissociation data for various systems consisting of methane/water/ethylene glycol and natural gas/water/ethylene glycol. A statistical thermodynamic approach, with the Cubic-Plus-Association equation of state, is employed to model the phase equilibria. The hydrate-forming conditions are modelled by the solid solution theory of van der Waals and Platteeuw. The thermodynamic model was used to predict the hydrate dissociation conditions of methane and natural gases in the presence of distilled water or ethylene glycol aqueous solutions. Predictions of the developed model are validated against independent experimental data and the data generated in this work. A good agreement between predictions and experimental data is observed, supporting the reliability of the developed model.     

Rational Design,Synthesis, and Optical Properties of Film‐Forming,Near‐Infrared Absorbing,and Fluorescent Chromophores with Multidonors and Large Heterocyclic Acceptors

A new series of film‐forming, low‐bandgap chromophores ( 1 a,b and 2 a,b ) were rationally designed with aid of a computational study, and then synthesized and characterized. To realize absorption and emission above the 1000 nm wavelength, the molecular design focuses on lowering the LUMO level by fusing common heterocyclic units into a large conjugated core that acts an electron acceptor and increasing the charge transfer by attaching the multiple electron‐donating groups at the appropriate positions of the acceptor core. The chromophores have bandgap levels of 1.27–0.71 eV, and accordingly absorb at 746–1003 nm and emit at 1035–1290 nm in solution. By design, the relatively high molecular weight (up to 2400 g mol^?1) and non‐coplanar structure allow these near‐infrared (NIR) chromophores to be readily spin‐coated as uniform thin films and doped with other organic semiconductors for potential device applications. Doping with [6,6]‐phenyl‐C₆₁ butyric acid methyl ester leads to a red shift in the absorption only for 1 a and 2 a . An interesting NIR electrochromism was found for 2 a , with absorption being turned on at 1034 nm when electrochemically switched (at 1000 mV) from its neutral state to a radical cation state. Furthermore, a large Stokes shift (256–318 nm) is also unique for this multidonor–acceptor type of chromophore, indicating a significant structural difference between the ground state and the excited state. Photoluminescence of the film of 2 a was further probed at variable temperatures and the results strongly suggest that the restriction of bond rotations certainly helps to diminish non‐radiative decay and thus enhance the luminescence of these large chromophores.     

Gravitational baryogenesis

We show that a gravitational interaction between the derivative of the Ricci scalar curvature and the baryon-number current dynamically breaks CPT in an expanding Universe and, combined with baryon-number-violating interactions, can drive the Universe towards an equilibrium baryon asymmetry that is observationally acceptable.